Rhabdophane Th-Pb ages indicate reactivation of Mesoarchean structures in west Pilbara Craton during breakup of Greater India and Australia-Antarctica

Uranium-Th-Pb dating of phosphate minerals in very low-grade metasedimentary rocks from the Archean Pilbara Craton, Western Australia, has revealed a long history of deformation and fluid flow during the Paleoproterozoic. However, this technique has not detected evidence for fluid flow along craton margins during Phanerozoic rifting and breakup. We report the use of in situ Th-Pb geochronology of rhabdophane, a hydrous light rare earth element phosphate, to date fluid flow in shale from the 2.76 Ga Mount Roe Basalt from drill hole number 6 of the Archean Biosphere Drilling Program (ABDP6), northwestern Pilbara Craton. Thorium-Pb dating of rhabdophane in carbonaceous shale yields three main populations with weighted mean 208Pb/232Th ages of 152 ± 6 Ma, 132 ± 4 Ma, and 119 ± 4 Ma, which indicates phosphate growth up to 2.64 b.y. after deposition. The rhabdophane ages are coeval with three major breakup events in eastern Gondwana: separation of Southwest Borneo and Argoland from Australia (ca. 156–152 Ma), breakup of Greater India from Australia (ca. 140–135 Ma), and separation of Greater India/India from Antarctica (ca. 123 Ma). The proximity of drill hole ABDP6 to major Mesoarchean faults and shear zones on the craton margin, which are parallel to rift propagation and basin development, points to episodic reactivation of ancient crustal structures >2.8 b.y. after their formation. Our results also highlight the potential of rhabdophane as a U-Th-Pb geochronometer for dating low-temperature (<200 °C) fluid flow and hydrous alteration. The migration of Mesozoic fluids through Archean shales adds weight to questions about the origin of geochemical signals in ancient altered rocks and how to extract information about the early environment and biosphere.

Riftquakes: Recording and Modeling Seismic Signals of Rifting at Pine Island Glacier

<p>Nearly 50% of Antarctic ice discharge into the ocean occurs via iceberg calving (Depoorter et al 2013). Large tabular icebergs calve from ice shelves along large fractures called rifts, but the physics of rifting are poorly understood. How fast does rift propagation occur? Does the timing of rift fracture coincide with episodes of unusual ice motion? We investigate these questions using data from seismometers and GPS sensors deployed on Pine Island Glacier ice shelf (PIG) from January 2012 to December 2013 surrounding the calving of iceberg B31, which exceeded 700 km2 in size and calved in November 2013 along a large rift. Using TerraSAR-X imagery, we identify a large 7km-long rift that must have occurred between May 8 and May 11, 2012. We identify a large-amplitude seismic signal on May 9, 2012, which we attribute to the rifting event. The signal is broadband, containing energy at frequencies higher than 1 Hz and lower than 0.01 Hz, and exhibits pronounced dispersion characterized by high frequencies arriving before low frequencies. We use features of the May 9 “riftquake” to detect thousands of similar events, which we classify using K-shape clustering. We hypothesize that the observed signals are flexural gravity waves generated by a bending moment applied to the ice shelf during fracture. To test this hypothesis, we model the ice shelf as a dynamic beam supported by an inviscid, incompressible ocean. We find that the model reproduces observed riftquake waveforms when forced with a bending moment. We then use a Markov Chain Monte Carlo inversion to model representative events from each cluster of observed events. The inversion reveals that source durations on the order of seconds have the highest likelihood of explaining observed riftquake waveforms, suggesting that rifting occurs on elastic timescales. Finally, we locate the riftquakes and find that a swarm of events originating at the rift tip occurs just after the start of a period of acceleration at PIG, suggesting that the stress concentrations driving rift opening are influenced by changes in ice dynamics.</p>

Reconstruction of the entire rift architecture of Theistareykir Fissure Swarm (northeast Iceland): integration between extensive UAV and field surveys

<p>Reconstructing the origin and kinematics of structures along active rifts is essential to gain a deeper knowledge on rifting processes, with important implications for the assessment of volcanic and seismic hazard. Here we reconstruct the architecture of an entire rift, the 70-km-long Theistareykir Fissure Swarm (ThFS) within the Northern Volcanic Zone of Iceland, through the collection of an extensive amount of 7500 quantitative measurements along extension fractures and normal faults, thanks to the integration between Unmanned Aerial Vehicles (UAV) mapping with centimetric resolution through Structure from Motion (SfM) techniques and extensive field surveys with classical methods. Quantitative measurements, collected across a wide area during several campaigns, comprise strike, opening direction and amount of opening at extension fractures, and strike and offset values at faults, along 6124 post-Late Glacial Maximum (LGM) and 685 pre-LGM structures.</p><p>The extent of the area covered by our data allowed us to pinpoint differences in the structural architecture of the rift. From south to north: i) extension fractures and faults strike ranges from mainly N10°-20°, to N00-10°, to N30-40°; ii) the opening direction starts from N110°, reaches N90-100° in the center and amounts to N125° in the northernmost sector; and iii) the dilation amount is in the range 0.1–10 m, then 0.1–9 m and it finally reaches 0.1–8 m. We explore such differences as due to the interaction with the WNW-ESE-striking Husavik-Flatey transform fault and the Grímsey Oblique Rift (Grímsey lineament), and to the structural inheritance of older NNE- to NE-striking normal faults. The reconstruction of the stress field resulting from such data allows the interpolation of the σ<sub>hmin</sub> values, through the unpublished software ATMO-STRESS, prepared in the framework of the EU NEANIAS project, in order to plot and examine the strain field.</p><p>Furthermore, mechanisms of rift propagation and the relation between magma systems are here investigated through the analysis of 281 slip profiles of the main Pleistocene-Holocene faults. Our data show a mechanism of along-axis propagation of the rift outward from the volcano: in fact, north of the volcano, 75% of the asymmetric faults propagated northward, whereas south of the volcano 47% of the asymmetric faults propagated southward. This can be due to the combination between the development of faults following lateral dyke propagation outward from the magma chamber, and faults nucleation near the volcano as a consequence of the different crustal rock rheology produced by a higher heat flux.</p>

Theoretical modelling of ice shelf vibrations forced by ocean surface waves

<p>Seismic measurements show that ice shelves vibrate in response to ocean surface waves over a wide frequency range, from long swell to tsunami waves. The phenomenon of wave-induced ice-shelf vibrations has been linked to calving of large icebergs, rift propagation, icequake activity, and triggering of catastrophic disintegrations. I will present some recent advances in theoretical modelling of wave-induced ice-shelf vibrations, including coupling of the ice shelf/sub-shelf cavity to the open ocean, studying the influence of ice-shelf thickening and seabed shoaling towards the grounding line, simulating transient vibrations in response to incident wave packets, and incorporation of real ice-shelf and seabed geometries via the BEDMAP2 dataset. I will introduce the open-source software iceFEM, which contains many of the latest advances.<span> </span></p>

Interplay of near surface rift evolution and deep-seated lower crustal flow: New findings from fully quantified crustal-scale analogue models

<p>Here we present new results and findings from an analogue modelling series using an extension gradient to simulate continental rifting in rotational settings. We study the effect of a pressure-gradient driven, rift-axis parallel lower crustal flow on rift propagation. The dynamically scaled two-layer models represent a brittle upper and a ductile lower crust. To simulate different crustal set-ups, we use variable ductile/brittle ratios R<sub>DB</sub>, where increasing values indicate a hotter crust with the brittle-ductile transition at relatively shallower depth. An additional package of sand on one part of the model simulates tectonic loading to provoke a pressure-gradient driven lower crustal flow.</p><p>Several factors play a role in dynamic rift propagation such as extension rates, fault evolution and the interplay of vertical motions at the surface as well as model-internal rift-axis parallel horizontal flow. We combine surface and internal deformation analysis using stereoscopic Digital Image Correlation and Digital Volume Correlation applied on surface stereo images and XRCT images, respectively to obtain the fully quantified model deformation.</p><p>Our results show that rift propagation occurs in two consecutive stages: (i) bidirectional step-wise growth in fault length by linkage and (ii) unidirectional linear fault growth. Strain partitioning of bulk extension causes episodic alternative fault growth on conjugate rift margin faults. Over time, fault activity abandons rift boundary faults and migrates inward creating intra-rift faults. This process occurs segment-wise along the rift axis, where different fault generations are simultaneously active. We quantify increasing lower crustal flow parallel to the rift axis with increasing R<sub>DB</sub> as the result of tectonic loading. In return, such lower crustal flow causes vertical and horizontal motions at the surface expressed by dynamic topography and deformation features.</p><p>These results give insights into deformation processes of rifting and highlight the important role of extension gradients on fault growth and strain partitioning in segmented rotational rift systems. Rift-axis parallel lower crustal flow in rotational rift settings may be of relevance when dealing with restorations of 2D crustal seismic sections across rifts.</p>